Maybe I’m missing something, but why have a single piece PCB instead of individual parts with a 3D printed slot for the sensor and LED.
A few wires later and a model scale up and your chess board can be a different size.
I suppose one piece is easier and cleaner, just less flexible.
Ordering and wiring up 64 small individual PCBs, vs one PCB, is not a hard choice.
Can confirm—I made a smart chess set like this years back by soldering Hall effect sensors and wires manually to a wood board for 64 squares. Every new soldered connection feels exponential—totally makes sense they didn’t do it this way. I would’ve probably done 4 4x4 PCBs instead so a single damaged PCB could be swapped out.